1. Field of the Invention
This invention relates to dealkalised sheet glass. The invention also relates to a method of producing dealkalised sheet glass.
2. Background of the Related Art
It is known that glass, especially ordinary soda-lime glass is apt to weather when exposed to adverse environmental conditions. In particular, when sheets of ordinary soda-lime glass are exposed to a warm, humid, calm atmosphere, sodium ions at the surface of the glass are subject to hydrolytic attack, and this leads to deterioration in the light transmitting properties of the glass. The problem is particularly acute during storage (especially in hot countries) and transport (especially by sea) of stacked sheets of glass. In some circumstances, there can even be a reaction between contacting sheets which leads to their becoming firmly bonded together. It has also recently become apparent that sodium-rich glass used for facing liquid crystal displays can cause premature deterioration of the displays due to sodium poisoning. Further, there are many coatings which may be applied to glass for various purposes, and it has become apparent that the use of sodium-rich glass in many coated glass products presents certain disadvantages. It has been found that due to the presence of high proportions of sodium ions, such as are found in ordinary soda-lime glass, such coatings are sometimes insufficiently adherent to the glass, and that the ageing properties of the coated product are not as good as they might be. It has also been found that the presence of sodium ions tends to promote haze in the coated product, and this is particularly disadvantageous for transparent products to be used for glazing purposes.
Various solutions to this problem have been proposed. The use of glass of a special low-alkali composition has been suggested. This presents inconveniences in manufacture, though it may be justified for special products. It also adds appreciably to the cost of the glass. A further suggestion has been to apply a sodium-impervious coating of silica (SiO.sub.2) to ordinary soda-lime glass, but this is also rather expensive.
It has been proposed to manufacture sheets of ordinary soda-lime glass and then to subject the sheets to a treatment which results in the production of dealkalised glass. For example, for the manufacture of mirrors, British Patent Specification No 294,391 suggests using an annealing furnace to reheat polished plate glass sheets to the annealing point and then subjecting the sheets to the action of an acid gas. In the Examples, finished glass sheets are reheated to 600.degree. C. and exposed to an atmosphere containing sulphur dioxide for about 30 minutes. Relatively low temperature treatments are also known.
Such treatments result in a depletion of the alkali ion content in a thin surface layer of the glass. Typically such treatments are performed in such a way that the sodium ion concentration at a depth of a few hundred nanometers in unaffected by the treatment. It is convenient to relate the sodium ion concentration to the sodium content of the glass before any dealkalising treatment. Thus for a typical soda-lime glass, a sodium ion concentration of 100% may correspond to a sodium content of 12 to 14% (or thereabouts) calculated as Na.sub.2 O by weight of the glass. The sodium ion concentration at various depths in the surface layer of the glass can be analysed in known manner by a proton bombardment technique which results in the conversion of .sup.23 Na to .sup.20 Ne with the evolution of an alpha particle. By monitoring proton and resonance energies and alpha particle emission it is possible to derive the sodium ion concentration at any depth beneath the surface with a resolution of 15 nm, and the results can be plotted to give a stepped line of sodium ion concentration against depth beneath the surface. When this stepped line is smoothed out (compare lines X in FIGS. 4 and 5) it will be seen that the sodium ion concentration increases with depth in almost linear fashion from an assumed zero sodium ion concentration at the surface until the 90% sodium ion concentration depth is reached, whereafter the plotted line moves up to 100% sodium ion concentration asymptotically. If that plot were strictly linear, the 50% sodium ion concentration depth would be 0.56 of the 90% depth. In fact values of 0.51 to 0.54 times the 90% sodium ion concentration depth are typical for the 50% sodium ion concentration depths of prior art dealkalised glasses, and for such known glasses, the shapes of the plots of sodium ion concentration against depth are all substantially similar.
It will be appreciated that the resulting dealkalised state of the glass surface is unstable in that there will be a tendency for sodium ions to migrate from within the depth of the glass towards the surface in order to re-establish an ionic population distribution there which is close to that of ionic equilibrium throughout the mass of glass. It will be appreciated that there are various factors which will govern the time taken for such equilibrium to be substantially re-established, and among the most important of these are the temperature of the glass and the extent to which the sodium ion concentration has been depleted in the surface layers of the glass. It will be appreciated that a given extent of surface dealkalisation can be expressed in terms of the depth at which the sodium ion concentration has a value of, for example, 50%, and that under similar conditions, because of the similar ion population distributions in previously known dealkalised glasses, as evidenced by the similar shapes of the plots of sodium ion concentration against depth, the benefit of any known dealkalised glass having a given 50% sodium ion concentration depth will be lost over a similar period of time.